Nuclear Decommissioning Trust Fund (Tables)

9 Months Ended

Sep. 30, 2012

Nuclear Decommissioning Trust Fund Disclosure [Abstract]

Summary of aggregate fair values and unrealized gains and losses (including other-than-temporary impairments) for the securities held in the nuclear decommissioning trust fund

The following table summarizes the aggregate fair values and unrealized gains and losses (including other-than-temporary impairments) for the securities held in the trust funds, as well as information about the contractual maturities of those securities.

	As of September 30, 2012							As of December 31, 2011
(In millions, except otherwise noted)	Fair Value		Unrealized Gains		Unrealized Losses		Weighted- average maturities (in years)	Fair Value		Unrealized Gains		Unrealized Losses		Weighted- average maturities (in years)
Cash and cash equivalents	$	4	$	—	$	—	—	$	2	$	—	$	—	—
U.S. government and federal agency obligations	33		2		—		11	43		3		—		10
Federal agency mortgage-backed securities	63		3		—		23	63		3		—		23
Commercial mortgage-backed securities	6		—		—		29	7		—		—		28
Corporate debt securities	72		4		—		11	54		3		1		10
Equity securities	286		143		—		—	251		113		1		—
Foreign government fixed income securities	5		—		—		6	4		—		—		8
Total	$	469	$	152	$	—		$	424	$	122	$	2

Summary of proceeds from sales of available-for-sale securities and the related realized gains and losses

The following table summarizes proceeds from sales of available-for-sale securities and the related realized gains and losses from these sales. The cost of securities sold is determined on the specific identification method.

	Nine months ended September 30,
	2012		2011
	(In millions)
Realized gains	$	8	$	4
Realized losses	5		3
Proceeds from sale of securities	316		294